Patent GB-779453-A (GB)
Title: Improvements relating to devices for controlling electromagnetic energy at high frequencies
Inventor: TURPIE ALEXANDER
IPC: H01P5/04
Filed: 1953-12-05
Abstract: 779,453. Variable attenuators. FERRANTI, Ltd. March 3, 1955 [Dec. 5, 1953], No. 33885/53. Class 40 (8). A device for controlling microwave energy comprises input and output transmission lines coupled at a junction to two branch lines into which the input energy is coupled and from which it is reflected in variable amounts to the output line. The junction 11, Fig. 1, may be of the ring type the circumference of the ring being one and a half wavelengths at the centre frequency of the pass-band of the system and the various branches being spaced at intervals of #/4 around half the circumference. Energy from the input line 12 is divided equally between the two branches 13, 23 and is partly absorbed by matched dissipative loads 14, 24 and partly reflected back to the junction by two similar reactive tuning elements 17, 27. Owing to a difference in length of #/4 in the branches 13, 23 the reflected energy is coupled entirely into the output line 30 and no energy returns to the input line. The amount of energy dissipated is controlled by tuning means 18, 28 which are connected together in such a manner that the reflection coefficients of the reactive elements 17, 27 are always equal. The tuning elements may be replaced by conductive vanes arranged for insertion to a variable extent across the guides, leakage being prevented by choke flanges around the vanes. In a modification (Fig. 2, not shown), the ring junction is replaced by a magic T-junction the reactive tuning elements and dissipative loads being arranged in the symmetrical arms. The junction in the construction of Fig. 4 comprises a directional coupler 47 arranged to divide energy from the input line 48 equally between branches 71, 81 which are of equal length. Owing to the directional properties of the coupler and the quadrature phase delay imposed upon the coupled wave, all energy reflected in the branches 71, 81 passes into the output line 49 and none into the input line 48. The lines 71, 81 are coupled by directional couplers 73, 83 similar to the coupler 47 to further lines 72, 82 terminated at one end by matched dissipative loads 52, 62 and at the other by short circuits 75, 85. The proportion of energy dissipated in the loads is determined by the position of the ganged plungers 74, 84 which terminate the lines 71, 81. In an alternative arrangement the plungers 74, 84 are situated in the lines 72, 82 and the fixed short circuits in the lines 71, 81. In a modification of this construction which operates in a similar fashion (Fig. 3, not shown), the lines 71, 81 are terminated at equal distances from the coupler 47 by matched disspative loads. A variable proportion of the energy in the lines 71, 81 is reflected back to the coupler by a pair of identical shunt T- junctions situated one in each line at corresponding positions between the coupler and the loads. The T-junctions are tuned by ganged pistons in a manner similar to the junctions 17, 27, Fig. 1. The waveguides may be replaced by coaxial lines in all the constructions.